A Diversity of Worlds

The Voyager 1 spacecraft, after traveling about 4 billion miles into space, turned around and looked back home. From such a distance, the Earth appeared as a pale blue dot, a single point of light suspended in the vast blackness of space. If aliens from much more distant worlds were to look at our solar system, the Earth, if it could be seen at all, would seem even more tiny and faint. How could they know that dot of light represents a world teeming with life?

We face this problem when we search for life in other solar systems. As yet, we have no pictures of extrasolar planets; the evidence for their existence comes from the gravitational and spectral effects they exert on their host star. Over the next decade, however, space telescopes may begin to search for and provide images of Earth-sized planets orbiting distant stars. These telescopes include the European Space Agency’s COROT and Darwin missions, and NASA’s Space Interferometry Mission (SIM), Kepler, and Terrestrial Planet Finders. These missions may be able to tell us about the geology, chemistry, and atmosphere of terrestrial worlds in alien solar systems. Such information could help determine if planets are rich with life like the Earth, or dead, barren worlds where life never took hold.

In May 2007, Victoria Meadows, Principal Investigator for the Virtual Planetary Laboratory at the California Institute of Technology’s Spitzer Science Center, presented a lecture at NASA’s Jet Propulsion Laboratory. In part four of this six-part edited series, she explains how different types of worlds, even ones not like the Earth, can still be potential havens for life.

A Diversity of Worlds

A lecture by Vikki Meadows

“Not all life is going to look like life on Earth. There are so many different types of worlds out there, so many things that we never expected to see that we are now seeing. It behooves us to not assume we’re going to see a spectrum that looks just like the Earth’s. That would be nice, but it’s probably not going to happen. We have to consider that there will be a huge diversity of different types of worlds.

This artist’s illustration shows a system with a star much like our Sun and a gas-giant planet similar to Jupiter. Most of the more than 200 extrasolar planets known today are gas giants up to 4,000 times larger than Earth, and are thought to be uninhabitable for life as we know it. Image credit: NASA/JPL/CalTech/T.Pyle, SSC

So to address this diversity of worlds, which we’ve never seen yet but we’re pretty sure are out there, my team develops models of planets. We start with a model that tells us the chemistry and climate of the planet, and into that we input things like volcanism and other elements. We look at what goes in at the bottom, including life, and we look at what escapes out of the top. We run this until the planet comes to equilibrium with the star that it’s going around.

So these models help us generate environments of planets that could be physically and chemistry reasonable. Then they also let us generate a spectrum of what that planet would look like if I was looking at it with a telescope.

When we use our planet formation models, the simulation is a bit like bumper cars. You put a lot of rocks around a star, and then you let those rocks crash together and eventually form planets. Some planets are small, some are large, some end up water-rich, and some have almost no water at all. Using this model, we do produce Earth-like planets. However, a whole bunch of weird planets of different sizes and different water masses also get formed. So we don’t expect all the terrestrial planets out there to be just like our solar system.

And also, you can have planetary diversity in time. The planet you’re standing on right now is just one example of the Earth. The Earth has changed its face quite dramatically over the last 4.6 billion years. The atmospheric pressure of oxygen, the amount of oxygen in our atmosphere, has changed drastically over time. Modern day oxygen levels are relatively high, but back in the Proterozoic they were pretty low, and back in the Archaean they were non-existent. About 2.3 billion years ago, there was a rapid rise of oxygen in our atmosphere, and that allowed multi-cellular life to develop. The rise of oxygen in the atmosphere probably was due to bacteria working away at photosynthesis.

The Hertzsprung-Russell diagram plots spectral class vs. luminosity (brightness) of a large sample of stars. Our Sun’s luminosity is 3.9 x 1026 Joules/s. The plot spans a large range in luminosity from a fraction of our Sun’s brightness (0.01 times) to much greater (10,000 times) the strength of our Sun. Image credit: NASA

In our models, one of the things we’ve been working on is looking at Earths around other stars. We ripped the sun out of our solar system and put something else in, for example, an F star which is much hotter than our sun, or we put in a cooler K or M star.

The results from the M star were interesting. M stars have always been considered the low rent district of the galaxy. They were not thought to be good places for life, because the planet has to be right up against the star in order to get enough radiation to stay warm, and then it gets tidally locked, plus it’s more susceptible to flaring from the star. However, these stars are very plentiful. So if we want to increase our chances of finding life, it would be great if we could detect life there.

In our M star experiment, we put an Earth in orbit around a star called AD Leo. AD Leo is a very young M star; it’s very active; it flares a lot. We saw a drop off in the amount of ozone on the planet, but we also saw an increase in the amount of methane. That was exciting because methane in the presence of oxygen (or ozone, O3) is a good biomarker. So it turns out that for planets around M stars, many of the biosignature gases survive for longer in their atmospheres and are much easier to see.

Three simulated planets – one as bright as Jupiter, one half as bright as Jupiter and one as faint as Earth – stand out plainly in this image created from a sequence of 480 images captured by the High Contrast Imaging Testbed at JPL. The asterisk marks the location of the system’s simulated star. Image credit: NASA/JPL/CalTech

Looking in the mid-infrared, it’s the same thing. The methane signal is whopping big, so that’s easier to detect. But amazingly, even though the planet didn’t produce as much ozone, the ozone is still as detectable. That’s a slight of hand that occurs because ozone absorbs heat in our atmosphere and causes our stratosphere to heat up. When you have less of it, the top of our atmosphere cools down. When that cooling occurs, the ozone molecule is easier to see. This was one of these things that we only discovered using this synergistic model — you can drop your ozone abundance gradually, but the detectability of that feature doesn’t drop off very fast at all. It stays detectable all the way down.

On the Earth, there’s a compound called methyl chloride that is a product of biomass burning. It’s also produced by plankton in the ocean. So it is a biosignature, but on the Earth there’s so little of it it’s very difficult to detect in our spectrum. On an M star planet, it builds up. So this is another potential biomarker that we hadn’t been considering before that we might be able to see.

Using our models, we went back in time and had a look at the early Earth. We looked back to the Proterozoic, when we had one-tenth of the oxygen we currently have now, and yet the ozone was just as detectable. The Earth back then also had methane from methanogens, types of bacteria that generate methane. Methanogens were around even in the Archaean, and there probably would have been a fairly high level of methane that was pumped out into the atmosphere as a result.

It turned out that it was easier to detect life 2.3 billion years ago then it is right now. If you go back into the steamy mists of time, way back into the Archaean period when we didn’t have a lot of oxygen in our atmosphere but we had a lot more CO2, you can see strong effects from carbon dioxide, strong effects from methane, but no ozone. That means the planet didn’t have any oxygen on it. Is it less habitable? No. It’s still just as habitable. It still has life on it, it just doesn’t have life that has produced enough oxygen to be able to see it.

The Earth’s atmospheric spectra, taken by the Ames Airborne Tracking Sunphotometer, AATS-14. Gases such as oxygen, carbon dioxide, and water vapor are indicated. Click image for larger view. Image Credit: NASA/Ames Sunphotometer-Satellite Team

Another thing we looked at — and this was loads of fun — was a high carbon dioxide early Earth-like planet. This was the most unlike-Earth planet that we had modeled. We essentially swapped out Earth’s nitrogen-oxygen atmosphere and replaced it with carbon dioxide, and then we doubled it. So we created a 2 bar carbon dioxide atmosphere, and then we added for good measure another bar of nitrogen on top of it. After we created this massive atmosphere on this planet, we looked at the effects that would have on the spectrum.

That massive atmosphere scatters radiation much more effectively than the less massive atmosphere, and that Rayleigh scattering tells us we’re looking at a more massive atmosphere. So that’s very good, because remember that we need to know the mass and the greenhouse gases in order to understand the surface temperature.

Looking at the high carbon dioxide early Earth in the mid-infrared was the weirdest thing we’d ever seen. Mid-infrared is the heat radiation coming off the planet. We’re used to seeing planets that have a nice sloping infrared curve; even Venus and Mars have that. But our high carbon dioxide early Earth had an intensely spiky spectral line because all the carbon dioxide was just eating away at that spectrum.

We could see from spectra that our model planet had a hotter planetary layer than the Earth. This planet had a surface temperature that was about 30 degrees hotter than the Earth’s surface temperature. So those are examples of the types of planets we can model and the sort of things we can learn by just looking at these squiggly spectral lines.”